genirq: generic chip: remove irq_gc_mask_disable_reg_and_ack()

Any usage of the irq_gc_mask_disable_reg_and_ack() function has
been replaced with the desired functionality.

The incorrect and ambiguously named function is removed here to
prevent accidental misuse.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>